The present invention relates generally to a control routine for devices used to control the flow of petroleum fuel vapors between a carbon canister and a combustion engine.
In order to comply with state and federal environmental regulations, most motor vehicles are now equipped with a carbon canister installed to trap and store petroleum fuel vapors from the carburetor bowl and/or the fuel tank. With the canister, fuel vapors are not vented to the atmosphere, but are instead trapped in the canister and then periodically purged from the canister into the engine where they are burned along with the air-fuel mixture. A solenoid is typically used to control purging of the carbon canister.
The solenoid mechanism includes a plunger that is movable between an open position, wherein the outlet port is not blocked and purge air communicates with the carbon canister, and a closed position, wherein the outlet port is blocked. When the coil within the cylindrical solenoid mechanism is energized, the magnetic force of the coil will attract the plunger collar and draw it toward the coil causing the plunger to move within the plunger guide to the open position. This motion will release a valve cap from a valve seat and open the air outlet nipple. The solenoid valve for a vehicle carbon canister will stay open as long as the coil is energized.
A spring is installed in compression within the plunger to bias the plunger in a closed position. When the coil within the cylindrical solenoid mechanism is de-energized, the spring returns the plunger to the closed position, with the valve cap pressed tightly against the valve seat, and blocks the flow of air through the solenoid valve for a vehicle carbon canister. The solenoid valve for a vehicle carbon canister will remain closed as long as the coil remains de-energized.
A pulse width modulated signal (PWM) modulates the duty cycle to obtain a certain percentage of the period in an active mode (i.e., energizing the coil). The frequency of operation determines the total period and the average current applied to the coil of the solenoid. This current generates a magnetic field that activates the plunger to compress the spring from a normally closed position. The spring constant of the spring is chosen so that the closure force of the spring will be greater than the force of the air pressure on the plunger collar. This will keep the plunger in the closed position (not shown) when the coil is de-energized. However, the spring constant is also chosen so that the magnetic force of the coil will overcome the spring force when the coil is energized and keep the plunger in the open position. In this manner, the movement of the plunger is proportional to the duty cycle that is being applied to the solenoid.
A high frequency is typically applied to the solenoid to diminish noise and lower power consumption. However, high frequency hinders the linearity of the proportional function of the solenoid and increases the hysteresis of the system because the activation pulses are so close in time that the pulses tend to meld with each other. Furthermore, when high frequency is applied, the plunger does not have time to fully travel the distance between the fully closed position and the fully open positions. Instead, the plunger vibrates or xe2x80x9cdithersxe2x80x9d proportionally to the frequency. It is known to control dithering by using a current driver to generate a proportional function between the average current and the input duty cycle. However, this requires the measurement of average current in real time which is difficult to determine.
Thus, there is a need for an apparatus and method for accurately controlling the purging of a carbon canister that will minimize dithering when a high frequency is applied.
The above discussed and other drawbacks and deficiencies are overcome or alleviated by a method and apparatus for controlling a solenoid-actuated charcoal canister purge valve to control the flow of purge fuel that is supplied via the purge valve to a cylinder of an internal combustion engine. The method and apparatus measure a feedback voltage (Vfb) of the solenoid as an indirect measurement of the average current Iavg applied to the solenoid. A microcontroller registers and generates a preselected input duty cycle (Idc) for use in energizing the solenoid- actuated purge valve. The input duty cycle energizes the solenoid-actuated purge valve using the input duty cycle to generate an output duty cycle from a current driver. The output duty cycle energized the solenoid to open to thereby supply a quantity of purge fuel to the cylinder. The feedback voltage (Vfb) is measured from the solenoid-actuated purge valve, wherein the feedback voltage (Vfb) corresponds to a feedback duty cycle (DCfb). An error between the input duty cycle (Idc) and the feedback duty cycle (DCfb) is calculated. The error is received by a proportional integral derivative (PID) control routine which generates a compensated output duty cycle to the current driver based on the error calculated to compensate for any deviation. The compensated output duty cycle compensates for any deviation from a linear relationship between the input duty cycle (Idc) and feedback voltage (Vfb), wherein Vfb corresponds to a flow of purge fuel. The microcontroller employs a reset function that uses a programmed feedback voltage corresponding to a certain duty cycle to be applied to control the average current applied to the solenoid-actuated purge valve. The reset function uses a set of programmable variables that include variables selected to change a slope of a proportional curve (Idc vs. Flow) for controlling the opening point and a linear dynamic range of the solenoid.